1. Field of the Invention
The present invention relates to a method of selecting time slots in a time division switching network and an apparatus therefor, and more particularly to a multi-slot information processing method for preserving the sequence integrity of multi-slot information in a frame in a time division switching system which uses a plurality of time division switches connected in tandem.
2. Description of the Prior Art
Telephone voice at 64 kb/s is transmitted using a time slot (8 bits) at intervals of 8 KHz sampling frequency. Therefore, n.times.64 kb/s broad band information is transmitted using n time slots. In this case, the sequence integrity of n time slots in a frame becomes important. For example, if a communication terminal at 128 kb/s (e.g., still image terminal) is used, two time slots (2.times.64 kb/s) are require for such communication. A switching connection for such a communication is called a referred to as multi-slot switching. In multi-slot switching, information sent from an orginating side by dividing it into two time slots must be assembled correctly at a terminating side, thus the sequence of two time slots must be controlled so as not to become reversed at the terminating side. In addition, it is necessary for a switching connection in a communication using a general terminal not to divide information within a single frame into two frames. This is called "sequence integrity of multi-slots in a frame".
A control method for ensuring the sequence integrity of multi-slots in a frame is described, for example, in "A Multi-slot Call Connection Method for Broadband Digital Switching", pp. 13 to 18, SE84-131 of Report on Technical Study (Switching) by The Institute of Electronics and Communication Engineers of Japan. Specifically, at page 16, the time slot sequence integrity in a frame is detailed.
However, such conventional technique manages the time slot sequence integrity in a frame only with respect to a single exchange. In actual communication where a plurality of tandem connected exchanges are used, the conventional technique does not give consideration to reduction of a path blocking rate while preserving the sequence integrity of multi-slots in a frame.
Even if time division switches are constructed as a complete switch group in case of switching connection in units of one time slot, they become an incomplete switch group in case of switching connection of multi-slots while preserving the sequence integrity. Thus, it is important to reduce blocking rate in a digital communication network handling multi-slot information. The blocking rate changes greatly depending on the arrangement of multi-slot information on an incoming highway. Therefore, if selection of time slots at the time division switch of an upstream exchange is not proper, all the time slots at the time division switch at a downstream exchange, which preserve the sequence integrity in a frame, may be in a busy state even if idle time slots are present. Thus, a path blocking occurs.